Inkjet printers operate by sweeping a printhead with one or more inkjet nozzles above a print medium and applying a precise quantity of ink from specified nozzles as they pass over specified pixel locations on the print medium. One type of inkjet nozzle utilizes a small resistor to produce heat within an associated ink chamber. To fire a nozzle, a voltage is applied to the resistor. The resulting heat causes ink within the chamber to quickly expand, thereby forcing one or more droplets from the associated nozzle. Resistors are controlled individually for each nozzle to produce a desired pixel pattern as the printhead passes over the print medium.
To achieve higher pixel resolutions, printheads have been designed with large numbers of nozzles. This has created the potential for printhead overheating. Each nozzle firing produces residual heat. If too many nozzles are fired within a short period of time, the printhead can reach undesirably high temperatures. Such temperatures can damage and shorten the life of a printhead. Furthermore, widely varying printhead temperatures during printing can change the size of droplets ejected from the nozzles. This has a detrimental effect on print quality.
Printhead overheating is often the result of a high "dot density" during a single swath of the printhead. When making a swath, the printhead passes over a known number of available pixels, some of which will receive ink and others of which will not receive ink. The pixels that receive ink are referred to as dots. The "dot density" is the percentage of pixels in a swath that receive ink and thereby become dots. When printing many types of images, such as text images, dot densities are relatively low and do not cause overheating. More dense images such as photographic images, however, require a much higher dot density and create the distinct potential for overheating.
Another problem caused by printing high-density images is that there might be insufficient ink in the nozzle area of the printhead for printing the next swath. Over time, firing a nozzle when it has an insufficient supply of ink will destroy the nozzle.
Generally, prior art printers have dealt with both of these problems by pausing the printhead. Where excessive printhead temperature is a concern, a pause is utilized to allow the printhead to cool. Similarly, a pause is used to allow additional ink to flow into the nozzle area of the printhead.
Any significant pause in printing, however, can have undesirable effects on print quality. Random delays between swaths result in horizontal bands with hue shifts. This is because different hues are formed when wet ink lands on ink droplets of various dryness applied during previous, overlapping swaths. Even more significant hue shifts become apparent at start/stop boundaries when pausing in the middle of swaths.
Another way to address the problems of overheating and insufficient ink quantity is to slow the velocity of the printhead as it moves across the print medium. The most significant disadvantage of this tactic is that it consistently reduces throughput for all documents, regardless of their density. A somewhat better approach is to slow the printhead only during swaths that are predicted to cause overheating or low ink quantities. However, this makes drop alignment difficult. The horizontal position of an ink drop is determined partially by the horizontal velocity of the printhead as the ink drop is ejected from the printhead. Thus, it is very difficult to line up the dots from two different swaths if the swaths are printed at different printhead velocities.
Note that each of the problems noted above can also be the result of a slow stream of data from a host. Specifically, a slow data stream can require pauses or slowing of the printhead, causing the described degradations of print quality.